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1. what is MDOPE?

Recently, there have been more and more discussions on recyclable flexible packaging. Polyethylene single material is an indispensable part of recyclable flexible packaging options, and MDO film is an important technical direction for polyethylene single material packaging. Today we will talk about MDO related technologies, applications and materials. The full name of MDO is machine directionorientation, which is an extrusion follow-up process used to increase the orientation of the film in the MD direction, thereby improving the stiffness, optics, and barrier properties of the film. The MDO stretching equipment is completed by 8-12 sticks through four stages of preheating, stretching, annealing and cooling. Preheating: Controlling the temperature around the softening temperature of the raw materials can better help stretch without damaging the film; and because the film is not completely melted, smaller crystals can be formed during the stretching orientation process to achieve better optics. Stretching: The film orientation was controlled by adjusting the stretch ratio. It should be noted that the higher the stretch ratio, the better. At the critical point of stretching, the film properties will reach equilibrium or decrease. Annealing, cooling: affect the film preheating shrinkage, especially the bag flatness; and film optical properties. MDO machines are divided into two types: online and offline. At present, the mainstream chooses online machines. Because the film is stretched in the offline state, the film is thin in the middle and thick on both sides, which is wasteful in use. The online machine can measure the thickness and feed it back to the film making system, so that the film thickness after stretching is more uniform and the cutting edge is reduced.

Changes in the properties of 2. MDOPE films

The following model diagram illustrates the change in crystal structure with film stretching before the critical draw ratio. The unstretched film has lamellae (lamella) of different sizes and is freely oriented (Figure 2 A); the stretched film forms smaller crystals and the molecular chain segments are oriented in the MD direction so that the crystallinity in the MD direction becomes higher (Figure 2 B). After the critical stretching ratio, the molecular chain segment is further stretched in the MD direction, and the lamellar structure is elongated to form a lamb string-like crystal structure [2]. Because of the change in molecular morphology during the stretching process, the macroscopic properties of the film are improved. It is summarized as follows: 1. The value of blocking WVTR is related to the length of the path of water vapor molecules through the film; in general, water vapor molecules pass through the amorphous region and bypass the crystal region. Before the critical draw ratio, the crystal arrangement is more regular than before drawing, so that the diffusion path becomes longer, thereby improving the WVTR value. After the critical stretch ratio, the molecules are in a mutton string structure, and the crystal arrangement is parallel to the MD direction, which shortens the passage path of the water vapor molecules, resulting in a decrease in the WVTR value. It should be noted that stretch orientation does not completely change the absolute structure of the film; even at the maximum stretch ratio, the effect on the structure of the film does not exceed 5%, so the WVTR value does not change by an order of magnitude. 2. Optical film optical properties include haze, gloss and transparency. The optical properties are closely related to the surface flatness of the film as well as the internal crystal size. MDO stretching process, film softening, stretching and cooling, itself will make the surface flatness becomes better; secondly, the stretching orientation process will also form smaller crystals; these two factors make the optical properties of the film with the film stretch ratio increase and improve, until the critical value is reached. There are other properties of the film will not be analyzed one by one.

Application of 3. MDOPE film

Because the stretched PE film has the advantages of high stiffness, high heat resistance, high tensile strength and good optical properties, MDO stretched PE film is applied in a single material packaging to replace the PET or PA surface layer. It should be noted that the MDO stretched PE film still needs to be compounded with the heat seal layer PE film. Because the heat resistance of MDO stretched PE film is greatly reduced compared to PET or PA, the formulation of the heat seal layer PE film should also be redesigned to reduce the heat seal temperature of the overall film. The single material packaging technology of polyethylene is still booming. In addition to MDO, there are also BOPE, multi-layer co-extrusion containing barrier materials, high barrier/heat resistant coating and other technologies. I believe that in the near future, these schemes will become more mature and the market will become larger and larger. Let's look forward to the day when the single material packaging will blow out.